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CMOS, BSI Sensor or Stacked Sensor - What are the differences?
Reading time: 10 minutes - August 03, 2023 - by Markus Igel

Camera Basics #13 - CMOS, BSI Sensor or Stacked Sensor? What are the differences?

As a specialty retailer, we have a mission: to provide you with the best possible advice and help you better understand the differences between the various technologies that can be found on the camera market. That's why today we want to introduce you to the different sensor technologies. Especially the differences between the stacked sensor and the BSI sensor.

We are looking forward to your feedback to this article! Just send us an email to content@fotokoch.lt

The history of the different image sensors

The first sensors were based on the CCD array sensor and the Bayer (CCD) sensor, while modern camera sensors use CMOS system. The CCD sensor was once incorporated in a two-layer variant in the first digital cameras and video cameras and was mainly used in astronomy. Bayer sensors are photosensors covered with a color filter, thus having a checkerboard of different colored photocells. These sensors have 50% green, 25% red, and blue sensors.

Expertise for show-offs:

Incidentally, it is the green component that essentially determines the sharpness and resolution on a lens. Since the green color has one of the greatest influences on the contrast & brightness perception when viewed in gray tones (about 72%).

The modern Active Pixel Sensor (CMOS sensor) is based on semiconductor technology and measures the incident light. CMOS technology has made many things possible, such as further technical integrations like exposure control or contrast correction.

CMOS sensor of the common methods side by side

The CMOS sensor in detail

Previously, we discussed the different technologies of the last few years, but now we want to focus on the modern CMOS technology. These sensors are particularly well suited to perceive infrared radiation, there is also a separate scene here, which makes the cameras sensitive to this spectrum of light.

Advantages of CMOS sensors

  • smaller size due to saving of components, which are now connected to the sensor
  • lower power consumption
  • Possibility of high frame rates (frames per second)

Classic CMOS sensor

In a classic CMOS sensor, the metal interconnects are located on the front side, behind the color filters. Behind the metal interconnects are the photodiodes. This design has the disadvantage that the dynamic range is degraded by the interconnects and the sensor offers slower autofocus performance than the backside illuminated sensor would allow.

Backside Illuminated CMOS (BSI CMOS)

Here, the sensor structure is designed in such a way that the metal connections are applied to the rear of the sensor. This design offers a higher dynamic range while still being much faster in readout speed than the classic CMOS sensor. The sensor also offers better low light performance because the diodes no longer lose light to the metal interconnects. Due to the line-by-line readout, the rolling shutter effect is present, but the ISO noise is significantly improved.

Stacked CMOS (BSI Stacked CMOS)

Here, there are now two metal connections on the back, which ensure that the sensor can be read out faster. In combination with a large and fast DRAM, it is thus possible to get a considerable amount of images into the camera's buffer. Depending on how advanced this sensor is, the readout delay is reduced to almost zero, as was the case with the mechanical shutter. Did you know that Stacked CMOS technology has not been used in cameras for very long? Sony has only been using these image converters, among others, since 2015.

Advantage of the stacked technology

  • higher frame rate
  • no distortion when using the electronic shutter
  • higher precision for autofocus & exposure

Fujifilm's X-Trans technology

In the historical part, we explained the Bayer system with its color pixels arranged in a checkerboard pattern. In the X-Trans arranged color filters there is always a pixel combination of 4-6 green color filters, while in the Bayer sensor the technology alternates the color line by line. This technology can now also be found in the Bayer sensor system and enriches it with a Quad Bayer Array, where instead of one color field per line, 4 color fields are combined, which are then recombined by the software (pixel binning). These quad Bayer sensors are often used in smartphones, action cams or drones that have high megapixels.

The advantages of X-Trans technology are the reduction of moir? effects and the improved green tones, which also have an effect on sharpness.

The Canon Dual Pixel Sensor Model

Canon's technological development of the Dual Pixel Sensor puts two photodiodes under each microlens instead of one. This allows light to be measured at two different angles for greater raw flexibility. If you use Canon's own software, you can also shift the bokeh and sharpness, for example. Of course, in this case we are talking about minimal adjustments, but they are sometimes crucial.

Likewise, the software can also be used to subsequently reduce or change the flaring and ghosting of the image, since the photodiodes both recorded different information.

When using the Dualpixel Raws, the camera stores two images in one instead of one, offering a greater variety of possibilities. The only real disadvantage here is that the file size is twice as large.

Conclusion

With the current Stacked CMOS BSI technology, we sometimes almost reach the physical limits that the readout rate and the dynamics of the image converters can allow. When we look at modern sensors, it makes perfect sense to talk about the installed processors and the memory sizes of the caching components. Because these still give the cameras one or the other noticeable upgrade. Because the more artificial intelligence and video formats / codecs move into the cameras, the more power the installed processor needs.

If you have a BSI or a Stacked CMOS (Stacked CMOS BSI), you will have some advantages with your camera, which make the cameras all the more versatile.

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A little task for you:

Which sensor do you have? Just check your technical data. Especially if you have an older camera, you can also come to D?sseldorf with a memory card and look at a modern sensor and compare the pictures at home.

Show us your pictures on Instagram and/or Facebook and tag us @fotokochde! We are looking forward to your results!


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